Variable blower controllers, or linear power modules, are devices that use power transistors operating in their linear region to achieve a variable DC voltage across a motor. As a result of operating in a linear manner, the transistors dissipate significant power and produce heat. The heat is removed from the system using a heat sink which is typically placed directly into the air stream of the vehicle HVAC system.
The energy of a large voltage transient will increase the power dissipated by the transistor as the voltage across the transistor and the current through it increase. As a result, the temperature of the transistor will rise. In the event of multiple transients, or a single significantly large and long duration transient, it is possible for the transistor maximum temperature rating to be exceeded. The result in this case can be an inoperable or improperly operating device.
Why not shut the transistor off? If one can shut the transistor off during the transient event, the instantaneous power dissipated by the transistor will be zero, but another problem then typically arises. As the transistor is turned off, the inductive load still tries to push current through the transistor, hence a voltage build up will be seen across the transistor terminals. Depending on the size of the inductive load, the voltage build up across the transistor terminals can exceed the device rating, and thus may damage the device.